1. Field of the Invention
Embodiments of the present invention relate, in general, to inter-cell spectrum sharing in cognitive radio networks and particularly to an on-demand spectrum contention protocol employing interactive messaging.
2. Relevant Background
Cognitive Radio is an enabling technology that allows unlicensed radio transmitters to operate in the licensed bands at locations when that spectrum is temporally not in use. Based on cognitive radio technology, Institute of Electrical and Electronics Engineers (“IEEE”) 802.22, following a Federal Communication Commission (“FCC”) Notice of Proposed Rulemaking in 2004, is an emerging standard for Wireless Regional Area Networks (“WRAN”) aiming to provide alternative broadband wireless access in, among other places, rural areas. Cognitive radio operates on a license-exempt and non-interference basis in the Television (“TV”) band (between 47-910 MHz) without creating harmful interference to the licensed services, which include, among others, Digital TV (“DTV”) and Part 74 devices (e.g. wireless microphones).
In a typical deployment scenario, multiple WRAN cells, each of which comprises a base station (“BS”) and associated customer premise equipments (“CPE”), may operate in the same vicinity while coexisting with DTV and Part 74 devices. In order to effectively avoid harmful interference to these licensed incumbents, the set of channels on which the WRAN cells are allowed to operate could be quite limited. For example as shown in FIG. 1, residing within the protection contours of DTV 140 and wireless microphones 150, both WRAN1 110 and WRAN3 130 are only allowed to operate on channel A, while WRAN2 120 may occupy either channel A or B, assuming that in total only 3 channels (channel A, B and C) are available. If WRAN1 and WRAN3 (or WRAN1 and WRAN2) attempt to perform data transmissions on channel A simultaneously, mutual interference between these collocated WRAN cells could degrade the system performance significantly.
Two approaches to resolve such contentions are known in the prior art as Aloha and Carrier Sense Multiple Access (“CSMA”). Both Aloha and CSMA resolve spectrum contention by deferring packet transmission with random periods. In CSMA a transmitting data station detects another signal while transmitting a frame, stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to send that frame again.
In the Aloha system, nodes that need to transmit simply send out their frames as soon as they are ready. Normally this means that the first node to start using the radio has exclusive transmission rights for an indeterminate time period, meaning other nodes cannot transmit any words until the first node finishes. In order to avoid this problem, the ALOHAnet required that the nodes break down their messages into small packets and send them one at a time with gaps between them. This allows other nodes to send out their packets in between, meaning the nodes can share the medium at the same time. However, if two nodes attempt to start their broadcast at the same time, a collision could occur. In the Aloha system after sending any packet, a node listens to see whether its own message was sent back to it by a central hub. When the node receives its message back, it can move on to its next packet.
But when a node does not receive its packet back, meaning that something prevented the packet from arriving at the hub—like a collision with another node's packet, the node simply waits for a random period of time and then attempts transmission again. Since each node chooses a random time to wait, one of the nodes is the first to re-try, and the other nodes then see that the channel is in use when they attempt transmission, resulting in yet another random wait period for those nodes. Under most circumstances this technique would significantly decrease or even eliminate collisions, but when the network gets busy the number of collisions can rise dramatically to the point where every packet will collide, breaking down this approach.
Although avoiding harmful interference to licensed incumbents is of prime concern, another key design challenge to cognitive radio based WRAN systems, with the scenario illustrated above in mind, is how to dynamically share the scarce spectrum among the collocated WRAN cells so that performance degradation, due to mutual co-channel interference, is effectively mitigated. Moreover, it is important that an inter-network spectrum sharing scheme be used to maintain efficient spectrum usage, accommodate a large scale of networks with various coexistence scenarios, and provide fairness in spectrum access among the coexisting WRAN cells. These and other challenges are addressed by one or more embodiments of the present invention.